A Monoidal category is a category with an associated biendofunctor that has an identity,
which satisfies Mac Lane''s pentagonal and triangular coherence conditions
Technically we usually say that category is Monoidal, but since
most interesting categories in our world have multiple candidate bifunctors that you can
use to enrich their structure, we choose here to think of the bifunctor as being
monoidal. This lets us reuse the same Bifunctor over different categories without
painful newtype wrapping.